1. Field of the Invention
This invention relates to a control for an automatic dryer, and more particularly to a current limiting network for such a control.
2. Description of the Prior Art
Donald E. Janke discloses an electronic control circuit for a dryer in U.S. Letters Pat. No. 3,471,939, issued Oct. 14, 1969 and assigned to Whirlpool Corporation, in which a timer motor of a dryer control is rendered inoperative by a shunt connected SCR in the absence of a predetermined dryness condition which is sensed by a moisture sensing circuit. In circuits of this type it has been the practice to provide a power resistor as a current limiting means for the DC current through the SCR when the SCR is conductive. This power resistor is relatively large, for example 720 ohms, 14 watts, and generates a great deal of heat which prevents mounting of the resistor in direct contact with the printed circuit board which carries other associated circuit components. In order to prevent damage to the circuit board and to the other components from the heat generated by the resistor, it has been necessary to mount the resistor either above the printed circuit board with special terminals and standoffs, or in a separate location with electrical connections extended thereto by way of spade type harness connections or the like. Problems have also been encountered in obtaining adequate quantities of reliable resistors of this type at costs which would be low enough to allow utilization of these resistors in consumer products.
Other dryer control circuits also utilize an SCR triggered by a sensor circuit to control operation of a timer motor. For example, attention is invited to U.S. Pat. Nos. 3,707,776; 3,471,938; and 3,765,100. In each of these circuits, however, the SCR is connected in series with the timer motor and does not require a current limiting means in the circuit.
U.S. Pat. No. 3,621,293 discloses the connection of an SCR in series with a coil and a dryer control circuit to control the operative state of the coil. A resistor and a capacitor are connected in series with the SCR to aid the coil in maintaining associated switches closed during conduction of the SCR when the coil is only receiving half-wave power.